1. Field of the Invention
The invention relates to a method for controlling torque in a powertrain that includes an induction motor.
2. Background Art
Electric machines for use in automotive vehicle drivelines have been developed in recent years for use in hybrid electric vehicles, fuel cell vehicles, integrated starter/generator systems, and electric power-assisted steering systems. These design efforts relate principally to drivelines characterized by increased fuel economy, reduced engine exhaust emissions, and an ability to accommodate increased vehicle electrical loads.
These drivelines make use of multi-phase induction machines. This is due to the rugged nature and the low cost of induction machines, which make them feasible for automotive applications. Induction machines have advantages also for industrial applications other than automotive applications.
An automotive vehicle driveline that would include an electric machine requires the electric machine to be capable of operating with variable torque control over a wide speed range. A current state of the art design approach for drivelines that include electric machines is referred to as xe2x80x9cfield orientationxe2x80x9d or xe2x80x9cvector controlxe2x80x9d.
There are several known methods for implementing field-oriented torque control of an induction machine drive. The simplest and most often-used form is based on the slip/frequency relationship of the induction machine. This is called xe2x80x9cindirect field orientationxe2x80x9d (IFO). An example of a control technique using IFO control methods is disclosed in co-pending patent application Ser. No. 10/060,434, filed Jan. 30, 2002, entitled xe2x80x9cMethod for Controlling Torque in a Rotational Sensorless Induction Motor Control System with Speed and Rotor Flux Estimationxe2x80x9d, filed by Alexander T. Zaremba et al. That co-pending application is assigned to the assignee of the present invention.
A key feature of the IFO technique is its dependency on only two parameters of the induction machine; i.e., the slip gain and the torque gain. The slip gain is of special interest since its value directly affects the dynamics of the torque response, whereas the torque gain, if the controller is mistuned, would result only in a scaling of the resultant torque produced.
Because of the relationship between the dynamics of the torque controller and the tuning of the slip gain estimate, there is a need in drives that include an induction machine for tuning of the slip gain estimate during a calibration sequence or during the normal operation of the system. Furthermore, it is desirable to develop a tuning method that does not depend on other machine or system parameters, and which is capable of performing the tuning without requiring measurement of the torque being produced. This is of importance in automotive applications where high volume production would not permit individual testing of the electric machines for the purpose of tuning the controller for machine-to-machine variations in parameters.
The invention comprises a method and system for estimating the slip gain (or the rotor time constant) of an induction machine based upon its transient response to a change in an input torque command. The xe2x80x9cslip gainxe2x80x9d is obtained by computing the inverse of the rotor time constant. The slip is the excitation frequency of the stator flux with respect to the rotor speed. The difference between the two is equal to the slip.
Unlike certain known control systems that determine slip gain based on steady state behavior, the slip gain estimate of the present invention is based on the transient response of a commanded rotor flux reference frame stator voltage following a command for a change in torque. The method of the invention will determine the transient response of the stator voltage in an indirect field oriented induction machine drive. If the slip gain estimate is incorrectly tuned, the results of the slip gain estimate calculation will be compared to the transient response of the stator voltage when the slip gain estimate is correctly tuned. The difference between the stator voltages corresponding to an incorrectly tuned system and a correctly tuned system will be determined, and the slip gain estimate will be corrected based upon the magnitude of the difference.
This control is achieved without reliance on any other machine or system parameters, and it does not require torque measurements. It requires only a current sensor and a rotor position sensor, which normally are included in an electric machine drive. The method of the invention can be used during normal operation of the drive with the tuning function in the background or during a calibration sequence before the drive is used in a finished product.
In practicing the invention, the induction machine drive would have a rotor and a fixed stator that forms a torque flow path from a torque input to a torque output. The method of the invention comprises the steps of commanding a change in torque at the torque input, determining commanded q-axis and d-axis stator voltages, calculating the product of the stator voltage and a desired windowing function. The windowing function is used as a multiplier for stator voltage data following a change in a torque command to effect a correctly tuned slip gain estimate.
Parameters, called q and d factors, are calculated for a current transient event as a time integral of a windowed q and d axis stator voltage waveform. Using the q and d factors, a determination is made with respect to whether the estimate of slip gain is accurate. The slip gain estimate then is corrected for a given change in commanded torque, if necessary, based on the q and d factors relative to a slip gain estimate for a correctly tuned drive.
In another aspect of the invention, a system is provided for controlling torque which establishes a stator current in the rotor flux frame following a change in commanded input torque. The difference between stator current in the rotor flux frame and a commanded input stator current in the rotor flux frame are used with estimated rotor flux to develop a commanded stator voltage in the rotor flux reference plane.